1. Field of the Invention
The present invention relates to a method for molding a thermoplastic resin according to which the surface state of a mold can be faithfully transferred to the surface of molded articles.
2. Description of the Related Art
In the molding of a thermoplastic resin, the temperature of the mold is usually kept sufficiently lower than the temperature at which the molding resin is solidified. This is necessary for cooling, in a short time, a resin material that is very low in thermal conductivity and is in the molten state to a temperature at which the resin can be removed as a molded article. Furthermore, in order to faithfully transfer the surface state of the mold to the resulting molded articles, it is necessary to press a resin in the state of low viscosity to the mold under a high pressure. However, if the mold temperature is lower than the solidification temperature of the resin, filling of the resin and solidification of the resin proceed simultaneously, and the resin contacting the mold at its flow front is rapidly cooled and increases in viscosity. Furthermore, since the resin solidifies in the state of being pressed to the mold surface under a low pressure, it becomes difficult to faithfully transfer the surface state of the mold to the resulting molded articles. Therefore, in the case of usual injection molding, poor appearance of molded articles due to uneven gloss, weld lines, flow marks or jetting may result, or inferior transfer of fine pits in precision molded articles such as optical disks may result. Furthermore, short shot may occur in thin-wall parts.
In order to enhance the transferability of the mold surface, it is necessary to prevent or minimize the solidification of resin during the step of filling the resin.
In the case of injection molding or the like of thermoplastic resins, it has always been demanded to enhance economically the transferability of the mold surface without prolongation of molding cycle time. For enhancing the transferability of a mold surface, various methods have been proposed as exemplified below.
1. A method of repeating heating and cooling of the mold surface by passing alternately a heating medium and a cooling medium through the mold. (Plastic Technology, Vol. 34 (June), 150 (1988) and others).
2. A method of selectively heating the mold surface by radiofrequency induction heating just before molding. (U.S. Pat. No. 4,439,492 and others).
3. A method of providing an insulating layer and an electrically conductive layer on the mold surface and passing a current through the electrically conductive layer. (Polym. Eng. Sci., Vol. 34 (11), 894 (1994) and others).
4. A method of radiation heating the mold surface. (Gosei Jushi, Vol. 42 (1), 48 (1996) and others).
5. A method of coating the mold surface with a heat insulating layer and carrying out the molding with heating the mold surface by the heat of the molding resin per se. (U.S. Pat. No. 5,362,226, W097/04938 and others).
In the report of B. H. Kim (Polym. Plast. Technol. Eng., Vol. 25 (1), 73 (1986)), the above methods 1, 2, 3 and 4 which heat the mold surface by external energy such as electricity just before molding are called active control method and the method 5 which heats the mold surface with the heat of the molding resin per se without application of external energy is called passive control method.
Both of the active control method and the passive control method carry out the molding with heating the mold surface at the time of injection molding. That is, when the injected molten resin is pressed to the wall surface of the mold, the mold surface is heated to a temperature higher than the solidification temperature of the resin, whereby transferability of the mold surface is improved.
The present invention is a method of attaining its object by a mechanism which is utterly different from these conventional molding mechanisms to improve the transferability of mold surface. That is, a method for obtaining a remarkable effect by a new idea different from conventional techniques. The present invention has been accomplished on the basis of this finding.
Known prior art somewhat relevant to the present invention will now be explained.
The so-called counter pressure method comprises injection molding of a foamable resin containing a foaming agent or water in a pressurized state by injecting a pressurized gas into a mold cavity prior to filling of the resin, thereby avoiding surface defects such as swirl marks on the molded articles caused by blowing gas. According to this method, a gas pressure is previously applied to a mold cavity in order to prevent occurrence of surface defects due to bursting of foams produced by the blowing gas or vaporized water at the flow front of the molten resin flowing through the mold cavity. The gases used in this case may be those which cause no deterioration of resin due to oxidation. Air is generally used, and any of the inert gases can be used in this molding method. This counter pressure method is employed for injection molding of resins containing foaming agents or resins which have dried insufficiently. When the counter pressure method is employed for molding of generally unfoamable resins, the following problems occur, namely, the gas present in the cavity enters between the molten resin and the mold to hinder the transfer or, in the case of the gas being air, the air is in the state of high oxygen concentration at high temperatures in the part where the air is compressed by the resin in the cavity, whereby deterioration of the resin due to oxidation is brought about. Thus, there is no effect to enhance the transferability of mold surface. Therefore, in order to precisely and faithfully transfer the state of mold surface to the resulting molded articles, the mold is slightly opened only at the time of filling of resin to release the air in the cavity or the pressure in the mold is reduced by a vacuum pump.
JP-A-62-231715 discloses a method of injection molding a water-containing polymer alloy using counter pressure method, and refers to inert gases such as air, nitrogen and carbon dioxide as a gas used for pre-pressurization of mold cavity, but it never suggests the idea of the present invention explained hereinafter.
Furthermore, JP-A-61-213111 discloses a reaction injection molding comprising mixing two monomers and injecting the mixture, wherein the molding is performed after the inner atmosphere of mold cavity is replaced with carbon dioxide of atmospheric pressure thereby to reduce voids produced by the air incorporated into resin at the time of filling the resin. However, the reaction injection molding in which the mold temperature is higher than the temperature of the raw material mixture of two or more monomers utterly differs in the technical field from the injection molding of thermoplastic resins according to the present invention, and thus the prior art does not disclose a method for improving inferior transferability of mold surface which is caused by solidification of resin during a step of filling the resin.
On the other hand, as shown in many literatures such as J. Appln. Polym. Sci., Vol. 30, 2633 (1985), it is known that when carbon dioxide is absorbed into a resin, this acts as a plasticizer for the resin and reduces the glass transition temperature, but this has not been widely applied to the molding of resins. As one of a few examples, DE-A-4314869 discloses a method in which carbon dioxide or a hydrocarbon in supercritical state is dissolved in a bioabsorbable polyester in a high pressure vessel to reduce the glass transition temperature thereof and the resin is molded at a low temperature of about 50.degree. C. However, since this method causes reduction of the glass transition temperature of the whole resin, it is necessary for the molding to use a mold temperature lower than usual temperature by the decrement in the glass transition temperature and thus there is no effect to prevent inferior transfer caused by solidification during the filling of resin.